As a measure to enhance the efficiency of solar cell modules and to secure their reliability over a long term from 20 years to more than 30 years, there have been made various reports and proposals with attention paid to encapsulant materials. In connection with enhancement of efficiency, it has been reported that silicones are advantageous, in internal quantum efficiency based on transmittance characteristics for light with wavelengths of around 300 to 400 nm, as compared with ethylene-vinyl acetate copolymer (hereinafter abridged to EVA) which is the main stream of encapsulant material at present (see, for example, S. Ohl, G. Hahn, “Increased internal quantum efficiency of encapsulated solar cell by using two-component silicone as encapsulant material,” Proc. 23rd, EU PVSEC, Valencia (2008), pp. 2693-2697 (Non-patent Document 1)). In addition, comparative experiments in output electric power between the cases of practically using EVA and silicones, respectively, have also been reported (see, for example, Barry Ketola, Chris Shirk, Philip Griffith, Gabriela Bunea, “DEMONSTRATION OF THE BENEFITS OF SILICONE ENCAPSULATION OF PV MODULES IN A LARGE SCALE OUTDOOR ARRAY,” Dow Corning Corporation (Non-patent Document 2)).
Originally, the use of silicones as encapsulant material was already achieved in the former half of the 1970s on the basis of manufacture of solar cells for space use. In manufacturing solar cells for ground use, however, there were problems as to the cost of silicones and as to workability in encapsulating the solar cells with silicones. In view of these problems, silicones were replaced by EVA which was available at low cost and in film form in those days.
In recent years, however, close attention has come to be again paid to enhancement of efficiency and long-term reliability of solar cells. At the same time, the performance of silicones as encapsulant material (for example, low modulus properties, high transparency, and high weatherability) has come to be highly evaluated again. Thus, a variety of new encapsulating methods based on the use of silicones have been proposed.
For instance, in JP-A 2009-515355 (Patent Document 1), there is proposed encapsulation with a hot melt type sheet composed mainly of an organopolysiloxane. However, it is difficult to process the material into a single-layer sheet form while maintaining high transparency. For example, in order to process the material into a thickness of around 1 mm, the processing method is limited to casting, and pressing, due to the “brittleness” of the material. Thus, the material is not suited to mass production. In addition, although the “brittleness” can be improved by enhancing moldability (formability) through admixing the material with filler, this approach is disadvantageous in that it becomes impossible to maintain high transparency. In JP-A 2007-527109 (Patent Document 2), there is proposed a process in which interconnected solar cells are disposed on or in a liquid silicone material which coats over a substrate by use of a multi-axis robot, after which the silicone material is cured to achieve encapsulation, without air bubble entrapment. In addition, in JP-A 2011-514690 (Patent Document 3), there is proposed a process in which solar cells are disposed on a cured or semi-cured silicone in a vacuum by use of a cell press having a movable plate, to achieve encapsulation without air bubble entrapment. On the other hand, in WO 2009/091063 (Patent Document 4), there is proposed a process in which an encapsulant material, solar cells, and liquid silicone material are disposed on a glass substrate, and a back surface protective substrate is finally laid over them to form a temporary laminated body, which is pressed into close contact in vacuum at room temperature, thereby achieving encapsulation. In this method, however, it may be difficult to develop the solar cell module to the practical-use size. In any one of these processes, a step of applying or disposing a liquid silicone material is involved before or after a solar cell encapsulating step. Quite unlike the currently adopted encapsulating process based on the use of EVA, this step greatly hampers the application of silicones as encapsulant material in the solar cell module manufacturing industry. In short, any one of the above-mentioned processes proposed in the patent documents differs greatly from the conventional solar cell encapsulating method, and may be impossible to deal with the current mass-production equipment.